This invention relates to improvements in lithographic dampeners, and particularly to the type of dampener described in U.S. Pat. No. 4,455,938 in which a dampening liquid is applied through a form roller, which is continually contacting a plate on the press, by a metering roller which is continually in contact with the form roller and the source of dampening liquid.
As described in that patent, a small reservoir of dampening liquid is formed in the nip above and between the form roller and metering roller, and the action of the two rollers is such as to mix or mill the ink and dampening liquid together to form an emulsion, which results in quick and uniform inking of the image areas of the plate and dampening of non-image areas. The ends of such nip are, in some embodiments, closed by seals which run tightly against the peripheral surface of the metering roller at its ends and against the end faces of the form roller. In another embodiment the metering roller carries dampening liquid to the nip from a pan, no seal is provided, and excess liquid at the nip flows outward and downward along the metering roller, which is longer than the form roller, returning to the pan.
In either case, those embodiments are somewhat disadvantageous, particularly at higher press speeds, since the friction from the tight seals increases with roller speed, as does seal wear. Further, if the press be operated at considerably different speeds, particularly if it is necessary to tighten the metering/form rollers' nip at higher operating speeds, the seals may leak as a result of their movement which is a consequence of increasing the nip pressure by moving the metering roller tighter against the form roller.
In the embodiment without seals there is a tendency to throw the liquid from the rollers onto adjacent press parts, instead of flowing along and with the longer end portions of the metering roller, and this will of course be more troublesome at higher roller speeds. Also the dampening liquid pan in that embodiment does not extend totally under the nip, so any collected liquid will tend to drip out of the dampener.
When the dampening liquid is supplied and replenished from a pan, in which the metering roller surface enters a pool in the pan, it is desirable to have some means for controlling the amount of liquid carried by the metering roller surface to the nip. In addition, especially printing either single color jobs with heavy ink deposit areas, or multi-color jobs with more than one heavy or bright color, there is a tendency with such dampeners to cause some ghosting of the printed image due to the repetitive return of ink in image areas, and dampening liquid in non-image areas, to the form roller and back to the metering roller. Thus, there is a need to avoid such repetitions or ghosting, and also to meter the amount of liquid carried on the metering roller from the pool to the metering/form rollers' nip.